This invention relates to powder coating compositions and more particularly to a coating powder composition capable of holding a high static electric charge and which forms a durable polymeric film coating upon heating.
Since their introduction in the early 1950's, powdered coating compositions and techniques have become increasingly popular in the coating industry. This is due to the several advantages which this coating method offers over conventional solvent-type coating methods. Today there are many coating powder compositions available on the market both in the thermoplastic and thermoset polymer categories. In spite of the fact that many polyamide and polyimide film coatings have excellent physical properties, polyamide and polyimide-producing powder coating compositions containing isocyanates are not, in general, available on the market. A principal reason for the lack of availability of such isocyanate-containing powder coating compositions is that the condensation reaction between isocyanates and compounds containing organic acids or acid anhydrides is rapid, particularly in the presence of a catalyst, and, thus, they do not exhibit good storage stability even in the powdered form. Moreover, isocyanates have a great propensity to react with water so that great care must be taken to prevent a powder coating material comprised of conventional isocyanates and organic acid or acid anhydride containing-compounds from coming into contact with moisture, even the moisture present in the air. Furthermore, the highly toxic nature of isocyanates has discouraged the general use of isocyanates in powder coating compositions. In spite of these drawbacks, there is a continuing need for new coating powders which have excellent physical properties even though they are made from isocyanates or isocyanate-type compounds.